01 - Preheat oven to 375°F. Line a baking sheet with parchment paper.
02 - Wipe mushroom caps with damp paper towel. Gently remove stems, setting caps aside. Finely chop stems.
03 - Heat olive oil in skillet over medium heat. Add chopped onion; cook 2–3 minutes until softened.
04 - Stir in minced garlic and chopped mushroom stems. Suté 2 minutes until fragrant.
05 - Add chopped spinach and cook until wilted, approximately 2 minutes. Remove from heat and let cool slightly.
06 - Combine cooled vegetable mixture with cream cheese, Parmesan, mozzarella, parsley, oregano, salt, and pepper in mixing bowl. Mix until thoroughly incorporated.
07 - Generously spoon filling into each mushroom cap. Arrange stuffed caps on prepared baking sheet.
08 - Mix breadcrumbs with Parmesan. Sprinkle evenly over stuffed mushrooms for golden crust.
09 - Bake 20–25 minutes until mushrooms are tender and tops are golden brown.
10 - Let cool 5 minutes to set filling. Serve warm.